drivers/watchdog: stm32: Disable IWDG_STM32_START_AT_BOOT by default

STM32 specific behavior "IWDG_STM32_START_AT_BOOT" is enabled
by default. As all vendor specific behavior, this is preferred
to be disabled by default and activated at application level,
so application can have better control of the driver.
For instance watchdog test can't run with this setting.
Disable by default.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>